In future optical networks, a large volume of information will be transmitted using existing optical fiber communication systems, and processing such as multiplexing and demultiplexing of a signal light, add and drop of a signal light, wavelength conversion, and switching, etc. will be used in a device such as an optical relay node and the like which will be provided at a point located far from a terminal apparatus. At that time, it will be effective to realize an optical network with the low power consumption in order to multiplex, demultiplex, and transmit information while reducing the number of conversion operations between an optical signal and an electrical signal as much as possible.
However, under these circumstances, at an optical relay node and the like in an optical network, conversion between an optical signal and an electrical signal is performed in the same way as in a terminal apparatus. In such a case, the transmitted optical signal is converted to an electrical signal first, the electrical signal is electrically processed, and then, the electrical signal is converted to an optical signal again. In this case, the circuit configuration is complicated and a larger amount of power is used in order to compensate for a loss due to photoelectric conversion.
On the other hand, in order to control an optical network, various types of information are monitored in real time at each point throughout the entire network, and effective network control is performed on the basis of a monitor signal. Also, in future information networks, the volume of information of each of various monitor signals is likely to increase considerably. Optical processing which does not employ photoelectric conversion allows an operation with a sufficiently broad band to be performed, as compared to processing of an electrical signal, regardless of the volume of information, and is, therefore, effective. Furthermore, a technology which enables efficient multiplexing and propagation of information existing at each point is very important in realizing a power conserving, flexible optical network.
Currently, for example, for transmitting monitor information, when monitor information is transmitted as an electrical signal separately from an optical signal, or even when monitor information is transmitted as optical information, a method in which electrical information is converted to optical information, and then, is written into a header part of a signal or the like, or a method using a dedicated optical wave, etc. is used.